for (let index of ['a', 'b'].keys()) {
	console.log(index); // 0 1
}

for (let elem of ['a', 'b'].values()) {
	console.log(elem); //a b
}

for (let [index, elem] of ['a', 'b'].entries()) {
	console.log(index, elem); // 0 "a"  1 "b"
}

